1. Field of the Invention
The present invention relates to a CAD (Computer Aided Design) system for use in designing wiring patterns of a printed wiring board or the like and a method for designing wiring patterns using a CAD.
2. Description of the Related Art
Thus far, when wiring patterns of a printed wiring board are designed with a CAD system, they are generated on a grid predetermined in the CAD system (the grid is hereinafter referred to as a basic grid) (wiring patterns generated on the basic grid are hereinafter referred to as grid patterns). The basic grid dimension is generally set to 2.54 mm, or 0.1 inch. There may be provided subgrids which divide the basic grid into smaller sections, for example 0.635 mm, 0.508 mm and 0.3175 mm.
However, as demands for small size and high integration of mounted components increase, the necessity of generating wiring patterns which cannot be placed on the basic grid arises. (Hereinafter, wiring patterns which cannot be placed on the basic grid are referred to as non-grid patterns.) These non-grid patterns have been generated by using a method where wiring patterns are generated on a grid with close dimensions. Another method is where horizontal or vertical start patterns are drawn from a terminal to any line in the X direction or the Y direction of the basic grid and then the following patterns are generated on the basic grid. By using such conventional non-grid pattern generation methods, the density of components mounted on printed wiring boards has been improved to some extent. However, since the kinds of components for use on printed wiring boards varies and components with different terminal intervals are used on one printed wiring board, a method where non-grid patterns can be further readily and flexibly designed is strongly desired.
In the method where the dimension of the grid is made closer to generate non-grid patterns, since optimum patterns are retrieved by using the basic grid, the retrieval efficiency is high. However, all of the terminals of the components to be used should be placed on the grid. Thus, the operator should design wiring patterns, while considering the grid. In addition, since the grid dimension is frequently changed, pattern defects, where intervals of patterns are not sufficient, may take place.
In another method where horizontal or vertical patterns are stored from terminals to at least one of a line in the X direction or the Y direction of the grid, when the shape of a component terminal is not round (for example, rectangular or oval), a line can be drawn to the direction along the shape of the terminal, namely in the horizontal direction or the vertical direction. However, when the shape of the terminal is round, since the drawing direction of the line is not always horizontal or vertical, the limitation of drawing direction degrades the operability of the system.